Why does soil absorb water

Soil water holding capacity Before we discuss the capacity of soils to hold water, we must understand the concept of capillarity. Capillarity Water molecules behave in two ways: Cohesion Force: Because of cohesion forces, water molecules are attracted to one another. Cohesion causes water molecules to stick to one another and form water droplets. Adhesion Force: This force is responsible for the attraction between water and solid surfaces.

For example, a drop of water can stick to a glass surface as the result of adhesion. Water also exhibits a property of surface tension: Water surfaces behave in an unusual way because of cohesion.

Since water molecules are more attracted to other water molecules as opposed to air particles, water surfaces behave like expandable films. This phenomenon is what makes it possible for certain insects to walk along water surfaces. Capillary action is demonstrated by the upward movement of water through a narrow tube against the force of gravity. Capillary action occurs when the adhesive intermolecular forces between a liquid , such as water, and the solid surface of the tube are stronger than the cohesive intermolecular forces between water molecules.

As the result of capillarity, a concave meniscus or curved, U-shaped surface forms where the liquid is in contact with a vertical surface. Capillary rise is the height to which the water rises within the tube, and decreases as the width of the tube increases.

Thus, the narrower the tube, the water will rise to a greater height. Capillarity is the primary force that enables the soil to retain water, as well as to regulate its movement. The phenomenon of capillarity also occurs in the soil. In the same way that water moves upwards through a tube against the force of gravity; water moves upwards through soil pores, or the spaces between soil particles.

The height to which the water rises is dependent upon pore size. As a result, the smaller the soil pores, the higher the capillary rise. Finely-textured soils, like in Maui, typically have smaller pores than coarsely-textured soils.

Organic matter is the material left behind by plants and animals. It also includes secretions from the roots of plants. Organic matter can absorb and hold lots of water but even in fertile soils there is only a small percentage of water-holding organic matter in the soil, especially when compared to the amount of clay in the soil. Top soil is typically very permeable because it is constantly being puncutured by roots and traversed by organisms such as worms.

Water drains quickly through top soil but much more of it is absorbed by layers of clay found deeper in the soil. The capacity of the soil to hold water against the pull of gravity is called its water holding capacity. Because clay is so fine and has such a high surface area per unit of volume it can absorb huge amounts of water.

Top soil may contain organic matter that can absorb water but the surface soil has more large pores and holes than it does microscopic ones. To hold more water a soil needs fine materials which create tiny spaces for water to cling to. A soil's permeability is determined by the relative rate of moisture and air movement through the most restrictive layer within the upper 40 inches of the effective root zone. Water and air rapidly permeate coarse soils with granular subsoils, which tend to be loose when moist and don't restrict water or air movement.

Slow permeability is characteristic of a moderately fine subsoil with angular to subangular blocky structure. It is firm when moist and hard when dry.

Water-holding capacity is controlled primarily by soil texture and organic matter. Soils with smaller particles silt and clay have a larger surface area than those with larger sand particles, and a large surface area allows a soil to hold more water. In other words, a soil with a high percentage of silt and clay particles, which describes fine soil, has a higher water-holding capacity. The table illustrates water-holding-capacity differences as influenced by texture.

Organic matter percentage also influences water-holding capacity. As the percentage increases, the water-holding capacity increases because of the affinity organic matter has for water. Water availability is illustrated in the figure by water levels in three different soil types. Excess or gravitational water drains quickly from the soil after a heavy rain because of gravitational forces saturation point to field capacity. Plants may use small amounts of this water before it moves out of the root zone.

Available water is retained in the soil after the excess has drained field capacity to wilting point. This water is the most important for crop or forage production.

Plants can use approximately 50 percent of it without exhibiting stress, but if less than 50 percent is available, drought stress can result. Unavailable water is soil moisture that is held so tightly by the soil that it cannot be extracted by the plant.